Radiofrequency coils have been developed that can be used as intravenous or arterial catheters. These catheters were placed in the right ventricle of anesthetized dogs via the jugular vein under fluoroscopic observation. 31P and H1 NMR signals were then obtained from these coils within the right ventricle. 31P NMR signals were detected from creatine phosphate, inorganic phosphate and adenosine triphosphate with adequate signal to noise within five minutes. This technique will be extremely useful in investigating the in vivo energy metabolism of internal organs with minimal surgical intervention.